Installing a replica domain controller
In Installing an AD forest root domain, you installed AD on DC1. If you have just one DC, then that DC is a single point of failure. With a single DC, when DC1 goes down, you cannot manage AD using the PowerShell cmdlets, and users cannot log on. It is always best practice to install at least two DCs. If you are using VMs for your DCs, you should also ensure that each DC VM is on a separate virtualization host.
To add a second DC to your domain, you run Install-ADDSDomainController on another host, for example, DC2. This cmdlet is similar to Install-ADDSForest in terms of parameters. As with creating your first DC, it is useful to carry out some tests to ensure the second DC's promotion can succeed.
In this recipe, you promote a host, DC2, to be the second DC in the Reskit.Org domain. Like when creating your first DC, after you promote DC2 to be a DC, you need to reboot the server before processing. And after the reboot, it is useful...
